1. Field of the Invention
The present invention relates to an inorganic fine particle-containing composition, an optical film, an antireflection film excellent in the antireflectivity and visibility, and a polarizing plate and a polarizing film using the same, suitably usable for a display device such as liquid crystal display, plasma display and CRT, particularly for a liquid crystal display.
2. Description of the Related Art
In a display device such as cathode ray tube display device (CRT), plasma display panel (PDP), electroluminescence display (ELD) and liquid crystal display device (LCD), an antireflection film is generally disposed on the outermost surface of the display for preventing reduction in contrast due to reflection of outside light or projection of an image and reducing the reflectance by using the principle of optical interference.
Such an antireflection film can be produced by forming a low refractive index layer having an appropriate thickness on the outermost surface and depending on the case, appropriately forming a high refractive index layer, a medium refractive index layer, a hard coat layer, an antiglare layer, an antistatic layer, a light-diffusing layer and the like between the support and the low refractive index layer. In order to realize a low reflectance, the material for the low refractive index layer is demanded to have a refractive index as low as possible. Furthermore, since the antireflection film is used on the outermost surface of a display, high scratch resistance is required. For realizing high scratch resistance of a thin film having a thickness of around 100 nm, strength of the film itself is necessary.
The refractive index of a material may be decreased by using a fluorine-containing polymer as the binder resin, but this tends to impair the film strength and decrease the scratch resistance. Thus, it has been difficult to satisfy both low refractive index and high scratch resistance at the same time.
A technique of incorporating an appropriate amount of an inorganic oxide fine particle into the low refractive index layer comprising a fluorine-containing polymer to increase the hardness of the film surface and improve the scratch resistance has been proposed. This technique is effective in reducing the reflectance and improving the scratch resistance but has a problem that aggregation of inorganic oxide fine particles occurs in the low refractive index layer and the film surface state changes for the worse.
For more reducing the reflectance or projection of an image or imparting hard coat property or dust resistance, a high refractive index layer, a medium refractive index layer, an antiglare layer, a hard coat layer, an antistatic layer and the like are sometimes appropriately formed between the low refractive index layer and the support. In general, a method of incorporating an inorganic oxide fine particle into a layer is employed as means, for example, for adjusting the refractive index of the layer, forming irregularities on the layer surface, increasing the layer hardness or imparting electrical conductivity. However, similarly to the above, there is a problem that aggregation of inorganic oxide fine particles occurs in the layer and the film surface state changes for the worse.
In order not to allow for aggregation of inorganic oxide fine particles in an organic solvent, it is generally important that the inorganic oxide fine particles are stably dispersed in the organic solvent. More specifically, the control of hydrophilicity/hydrophobicity or steric hindrance on the surface of the inorganic oxide fine particle is important and a technique of surface-treating the inorganic oxide fine particle by using an alkoxysilane is known. For example, a method of dispersing an inorganic particle in an organic solvent by using a silane coupling agent is described in Ganryo Bunsan Gijutsu Hyoumenshori to Bunsanzai no Tsukaikata oyobi Bunsansei Hyoka (Pigment Dispersion Technology How to Use Surface Treatment and Dispersant and Evaluation of Dispersibility), Gijutsu Joho Kyokai (compiler) (1999). Also, JP-A-2000-9908 (page 3), JP-A-2001-310423 (page 3) and JP-A-2001-100013 (page 3) disclose that when the inorganic fine particle is previously surface-treated, the scratch resistance or layer strength is improved. Furthermore, JP-A-2001-272502 (page 3) describes the effect on storage stability against aggregation of the inorganic fine particle in a coating solution. However, in the drying process of forming an optical functional layer after coating a coating solution for the formation of an optical functional layer containing an inorganic oxide fine particle, the organic solvent volatilizes to increase the concentration of the inorganic oxide fine particle and accelerate the aggregation. These techniques are still insufficient in the light of stability of the particle dispersion in the optical functional layer formed.
On the other hand, as mechanical means for improving the dispersibility, there is known a method of applying an ultrasonic treatment to a solution having dispersed therein fine particles, and then coating and drying the liquid dispersion to form a functional fine particle-containing layer (JP-A-2001-327917 (pages 4 and 5)). This method is surely effective for improving the dispersibility in a liquid dispersion before coating but has almost no effect in the light of improving the fine particle dispersibility in the formed layer, because similarly to the above, the organic solvent volatilizes in the drying process and the concentration of the inorganic oxide fine particle is increased.